This invention relates to the transmission of digital data, particularly the transmission of digital data that represents video signals.
An HDTV transmission system will provide HDTV images with much greater resolution than the existing NTSC (National Television System Committee) television (TV) image. However, any HDTV transmission system is currently required to use the existing NTSC TV 6 MHz channel allocations, which are in the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands. As a result, an HDTV image, which contains more picture information than an NTSC image, to provide the increased resolution, must be compressed in order to fit within an NTSC TV channel. Since much more powerful video compression schemes can be implemented with digital signal processing than with analog signal processing, it is possible that some form of digital transmission system will be required for an HDTV image so that the above transmission constraint can be met. However, there has been some concern about becoming committed to an all-digital transmission system because of the potential sensitivity of digital transmission to small variations in signal-to-noise ratio at the various receiving locations. This sensitivity can result in a quick degradation in performance, which is generally not considered acceptable by the TV broadcast industry. By comparison, the degradation in performance for presently used NTSC TV transmission is much more graceful.
The co-pending, commonly assigned U.S. patent of H. Y. Chung et al. entitled "Multiplexed Coded Modulation with Unequal Error Protection," Ser. No. 07/627,156, U.S. Pat. No. 5,214,656, filed on Dec. 13, 1990, discloses an advantageous technique for overcoming the shortcomings of standard digital transmission by providing for graceful degradation of over-the-air broadcasting of digital TV signals. Specifically, a digital HDTV signal is characterized into classes of "more important" and "less important" information (i.e., different classes of information), which are then transmitted during a time-frame by time-division-multiplexing between different coded modulation schemes for each class of information. Each time-frame comprises a number of time-slots, each of which is particularly assigned to either the more important information or the less important information. The particular coded modulation scheme for each class of information is chosen so that the more important information is provided with a higher amount of error protection than the less important information. This approach provides unequal error protection for the more important information, and allows a graceful degradation in reception quality at the TV set location because, as the bit-error rate at the receiver begins to increase with increasing distance from the broadcast transmitter, it will be the less important information of the TV signal information that will be the first affected.
In the time-division-multiplexing communications system disclosed above, the overall amount of information that is transmitted in a time-frame is dependent upon how many time-slots are assigned to the more important information and the less important information. Typically, the time-slots that are assigned to the less important information carry more image data, although with less error protection and, conversely, the time-slots that are assigned to the more important information carry less image data, although with higher error protection. Unfortunately, the overall amount of picture information in an HDTV signal is generally proportional to the complexity of each HDTV image that is transmitted, i.e., the amount of picture information that is considered to be more important and less important is variable. As a result, it may be desirable to increase the amount of more important information that can be transmitted. This is achieved in the above-mentioned Chung et al. approach by increasing the number of time-slots that are assigned to the more important information in any time-frame. Consequently, there are fewer time-slots available for the less important information and either some less important information must be dropped (with, hopefully, only a small decrease in picture quality), or the channel capacity must be increased by increasing the amount of less important information that is transmitted in each time-slot assigned to the less important information. The latter generally results in an increase in the size of the signal point constellation used to represent the less important information. Unfortunately, as the size of the signal point constellation increases either the average power level required to transmit the constellation increases, or, if the power level is constrained by system requirements, the amount of error protection provided by the constellation decreases.
Before proceeding with a description of an illustrative embodiment, it should be noted that the various digital signaling concepts described herein--with the exception, of course, of the inventive concept itself--are all well known in, for example, the digital radio and voiceband data transmission (modem) arts and thus need not be described in detail herein. These include such concepts as multidimensional signaling using 2N-dimensional channel symbol constellations, where N is some integer; trellis coding; fractional coding; scrambling; passband shaping; equalization; Viterbi, or maximum-likelihood, decoding; etc.